Fatigue threshold studies in Fe, Fe-Si, and HSLA steel: Part I. Effect of strength and surface asperities on closure

Yu, W.; Esaklul, Khlefa A.

doi:10.1007/bf02644562

Cited by 37 publications

(7 citation statements)

References 21 publications

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“…From recent results [7,10,12,14,15] it is evident that the threshold stress intensity range, AKth is a strong function of grain size and there is clear evidence [14,15] that this occurs because of the higher magnitudes of Kcl,t h at large grain sizes. It has been noted that Kd,th strongly increase with increasing grain size in Ni-base superalloys [7] and low strength steels [14,15].…”

Section: Introductionmentioning

confidence: 99%

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A theoretical model for roughness induced crack closure

Ravichandran

1990

Int J Fract

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A theoretical model for the effects of grain size on the magnitude of roughness induced crack closure (RICC) at fatigue crack growth threshold has been proposed. With the basic configuration of a crack propagating incrementally along planar slip bands and deflecting at grain boundaries, an idealized zig-zag crack path is considered. The effective slip band length is considered to be equal to grain size. It is assumed that the dislocations emitted from the crack tip upon loading to form the pile-up are completely irreversible to produce a combined mode I and mode II displacement at the crack tip. The assumption of continuously distributed dislocations in the pile-up facilitated the calculation of crack tip sliding displacement (CTSD) along the slip plane from which the mode I closure disregistry just behind the crack tip can be calculated. The closure stress intensity factor at threshold, Kcl,t h could then be expressed as a function of critical resolved shear stress, average macroscopic yield stress, angle subtended by the slip plane with the crack plane and the length of the slip band. Comparisons of the predicted trends with experimental data from various alloy systems indicate good agreement. NomenclaturePICC = Plasticity induced crack closure U = OICC = Oxide induced crack closure RICC = Roughness induced crack closure U~, U u = AK~h = Alternating stress intensity range at 0 = threshold hk/ Tcrss = Kcl,t h = Closure stress intensity level at threshold r 0 = K,~ax = Maximum stress intensity factor in a fatigue cycle ~l(x), q(x') = Dislocation distribution functions CTOD = z,, = Applied shear stress CTSD = b = Burger's vector E = G = Shear modulus a~. = v = Poisson's ratio 6,n~x = 1 = Length of the slip band or pile-up 6~ = N = Number of dislocations in the pile-up d =

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Section: Introductionmentioning

confidence: 99%

“…It has been noted that Kd,th strongly increase with increasing grain size in Ni-base superalloys [7] and low strength steels [14,15]. In the present study, a theoretical model for the variation of Kd, th with grain size is developed utilising fundamental characteristics of fatigue crack propagation along slip bands.…”

Section: Introductionmentioning

confidence: 99%

A theoretical model for roughness induced crack closure

Ravichandran

1990

Int J Fract

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“…This higher COD value is due to the influence of the wedging action of the wake zone. The physical reasons for the wedging action in the wake zone and its effects on FCP and cyclic loading conditions have been extensively investigated; for example plastic stretch in plane stress regions [17][18][19][20], fracture surface roughness [21-241, oxides [25,26], pores and shear lips geometry [27] or corrosion products [28] have been documented as causing a wedging action between the 898 G . MARCI Fig.…”

Section: Discussion Of Experimental Results In Light Of Present Undermentioning

confidence: 99%

NON‐PROPAGATION CONDITIONS (ΔK_th) AND FATIGUE CRACK PROPAGATION THRESHOLD (δK_T)

Marci

1994

Fatigue Fract Eng Mat Struct

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Fatigue crack propagation threshold values have been determined with two experimental methods, it., the constant R method and the constant K, , method. Three materials, namely A17075-T735 1 and Ti6Al4V STA in the LT-and TL-orientations, and a Ti-turbine disk material (IMI 685) in the CR-orientation, were investigated.The paper is divided into 3 parts. In the first part the test conditions, the experimental results and the conclusions drawn from the experimental results are presented, namely that the three different functional dependencies of AK* on R cannot be reconciled with present continuum mechanics concepts. In the second part, some facts used in conjunction with the da/dN -AK,, methodology are applied to the nonpropagation condition AKth. Parameters such as KO,, the threshold AKT, and a parameter "K,; are investigated by numerical modelling of their individual in3uence on the AKth versus R curves. This modelling work shows that the individual AKth versus R curves are primarily dependent on the KO, behavior of the respective material. Further, it is shown that the threshold AKT is a constant value, independent of any particular cyclic loading condition. In the third part of the paper, the AK,, concept is applied to the experimental results obtained in the first part. Using either experimentally or semiempirically determined KO, functions and the measured AKr values, the A K~ versus R curves of the three materials investigated were accurately reconstructed. It follows that the AKth versus R curves of the individual materials are the natural consequence of the driving force for fatigue crack propagation, namely AKem NOMENCLATURE FCP = fatigue crack propagation F-, F, , , = minimum and maximum load applied during a cycle K,,, K,,, = minimum and maximum stress intensity factor of a cycle AK = Kmx -Kmin = stress intensity range AKth = stress intensity range when FCP just ceases AKr = materal property describing the maximum AK which causes no continuous FCP KO, = the opening instant of the stress intensity factor under arbitrary cyclic loading K,,(max. value) = upper bound value of KO, at a given K,,, AKe, = effective part of AK front during a cycle F,,, K,, = minimum load and minimum stress intensity factor experienced by the crack KZaX = lowest K, , at which FCP occurs under zero-tension cycling R = Kmin/Kmax = stress intensity factor ratio R, = R-ratio at which Kmin = K,, and AK = AKT 4, ni = coefficients of polynomial equations 'Y(R), @ (Kmax) = polynomial functions

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“…(6) can be obtained from Eq. (7) by assuming the dominant obstacle is the grain boundaries and equating r, to the grain diameter, D. The AKoyaY4D-relation observed in Figure 28 has also been explained on the basis of roughness-induced crack closure [22] by Gerberich, et al [16]. In this case, the grain size dependence arises from crack closure due to contacts of the fracture surface asperities at a distance on the order of the grain size, D, behind the crack tip.…”

mentioning

confidence: 90%